Redundancy is an engineering method of improving system and equipment reliability. Mainly, redundancy consists in using extra units (subsystems, modules and/or additional elements) within the system to increase reliability. This kind of redundancy is usually called structural. Redundancy might be called functional when a system may perform the same operation by several different ways. For instance, a communication network may be able to by-pass its failed links or switches. Another possibility is time redundancy, where the system has extra time for possible repetition of the same operation after a failure. As an example, one can consider a computer system with restarting in the case of error. In this article, we only consider structural redundancy.
Redundancy may be implemented on the system or unit levels. By system-level redundancy, we mean that an entire system would be replaced upon its failure by an identical structure; by unit-level redundancy, we mean that an individual unit...
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